Bipolar transistors are electronic devices with two p-n junctions that are in close proximity to each other. A typical bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, the two p-n junctions, i.e., the emitter-base and collector-base junctions, are in a single layer of semiconductor material separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar-transistor action.”
If the emitter and collector are doped n-type and the base is doped p-type, the device is an “NPN” transistor. Alternatively, if the opposite doping configuration is used, the device is a “PNP” transistor. Because the mobility of minority carriers, i.e., electrons, in the base region of NPN transistors is higher than that of holes in the base of PNP transistors, higher-frequency operation and higher-speed performances can be obtained with NPN transistor devices. Therefore, NPN transistors comprise the majority of bipolar transistors used to build integrated circuits.
As the vertical dimensions of the bipolar transistor are scaled more and more, serious device operational limitations have been encountered. One actively studied approach to overcome these limitations is to build transistors with emitter materials whose band gaps are larger than the band gaps of the material used in the base. Such structures are called ‘heterojunction’ transistors.
Heterostructures comprising heterojunctions can be used for both majority carrier and minority carrier devices. Among majority carrier devices, heterojunction bipolar transistors (HBTs) in which the emitter is formed of silicon (Si) and the base of a silicon-germanium (SiGe) alloy have recently been developed. The SiGe alloy (often expressed simply as silicon-germanium) is narrower in band gap than silicon.
The advanced silicon-germanium bipolar and complementary metal oxide semiconductor (BiCMOS) technology uses a SiGe base in the heterojunction bipolar transistor. In the high-frequency (such as multi-GHz) regime, conventional compound semiconductors such as GaAs and InP currently dominate the market for high-speed wired and wireless communications. SiGe BiCMOS promises not only a comparable performance to GaAs in devices such as power amplifiers, but also a substantial cost reduction due to the integration of heterojunction bipolar transistors with standard CMOS, yielding the so-called “system on a chip.”
State-of-the-art NPN HBTs require a high unity current gain frequency fT and a high unity unilateral power gain frequency fmax. Base resistance, Rb, is an important factor that must be lowered in order to obtain a high-performance, e.g., high fT and fmax, HBT.
One solution for providing high performance HBTs is to build the HBT on a structure that includes a raised extrinsic base. The raised extrinsic base is formed by depositing an extra layer of polycrystalline silicon (or SiGe) atop a conventional SiGe extrinsic region.
In prior art process for forming HBTs containing raised extrinsic bases, elaborate and complicate processing steps are used to integrate a self-aligned raised extrinsic base into an advanced BiCMOS process. Thus, there is a need for providing a method that can easily integrate a self-aligned raised extrinsic base HBT into advanced BiCMOS processing.